1. Field of the Invention
The present invention refers to a coin-type (button-shaped) non-aqueous electrolyte rechargeable battery employing a substance capable of occluding and discharging lithium as the active material for the positive electrode and negative electrode, and a lithium ion conductive non-aqueous electrolyte, particularly, to a heat resistant non-aqueous rechargeable battery suitable for reflow soldering.
2. Background Information
The use of coin-type (button-shaped) non-aqueous rechargeable battery in the field of back-up power source for electronic devices is increasing because of its advantages such as high energy density, light weight, etc.
Most of the coin-type (button-shaped) non-aqueous rechargeable batteries known heretofore required an addition of lithium to the negative electrode in some form as an active material. For instance, in case of a battery using lithium-aluminum alloy for the negative electrode and a manganese oxide containing lithium of 3-V class for the positive electrode, lithium had to be pressure-welded to the negative electrode. In case of a battery using carbon for the negative electrode and a lithium-containing manganese oxide of 3-V class for the positive electrode, lithium had to be electrochemically inserted into the negative electrode.
In the batteries above, the selection of the material for the gasket which assures the air tightness and liquid tightness of the battery, as well as the insulation of the positive electrode and negative electrode cans is particularly important. Conventionally used as the gasket material is the inexpensive polypropylene, because of its advantages in resistance against chemicals, elasticity, creep resistance, and moldability which enables injection molding.
In case of using the batteries above mainly for memory back-up power sources, the terminals for use in soldering are welded to them, and are often soldered onto the printed wiring boards together with memory devices. Conventionally, soldering irons has been used for soldering the batteries to the printed wiring boards; however, with the recent requirements for more compact electronic devices which boast high functionality, there is a demand for increasing the number of electronic components to be mounted on the same area of the printed wiring board, and this made it difficult to afford a space for inserting the soldering iron. Furthermore, to reduce the production cost, automated soldering had been keenly demanded.
In the light of the aforementioned circumstances, there is employed a method comprising applying a solder cream and the like to the portion to be soldered and then mounting the components thereon, or a method comprising, after mounting the components on the board, supplying small beads to the soldering portions, and passing the thus mounted printed wiring board through a furnace whose atmosphere thereof is maintained at a high temperature set at a range of, for instance, 200 to 230xc2x0 C., thereby allowing the solder to melt and accomplish the soldering (this method is referred hereinafter as xe2x80x9creflow solderingxe2x80x9d). However, the coin-type (button-shaped) non-aqueous rechargeable battery of the conventional art had not been designed to use a heat-resistant material, and this disadvantageously led to the loss of its functionality as a battery during performing reflow soldering.
Most of the 3-V class coin-type (button-shaped) non-aqueous rechargeable batteries known heretofore employ a manganese oxide or a lithium-containing manganese oxide for the positive electrode and a material to which lithium is added in some form during the production process for the negative electrode. However, the material to which lithium is added in some form loses stability during the reflow soldering.
For instance, in a 3-V class coin-type (button-shaped) non-aqueous rechargeable battery using Li4Mn5O12 as the lithium-containing manganese oxide for the positive electrode and a lithium-aluminum alloy for the negative electrode, the lithium alloy reacts with the electrolytic solution in almost any type of combinations of electrolytic solutions and the heat-resistant battery members as to cause abrupt bulging or explosion.
Similarly, in case of a 3-V class coin-type (button-shaped) non-aqueous rechargeable battery using Li4Mn5O12 as the lithium-containing manganese oxide for the positive electrode and carbon doped with lithium either by contact doping or by electrochemical process for the negative electrode, the electrolytic solution undergoes reaction with the lithium-doped negative electrode as to cause abrupt bulging or explosion.
Furthermore, in a coin-type (button-shaped) non-aqueous rechargeable battery of the conventional type, no material resistant to the reflow temperature is used for the electrolytic solution, separator, or gasket. Thus, this lead to problems of causing, for instance, boiling or dissolution.
In the light of the aforementioned problems, the present invention utilizes a molybdenum oxide as the active material for the positive electrode. In particular, MoO3 was found to be favorable because it has a high potential, and hence it allows setting the battery potential at a high value. Furthermore, there has been found a heat-resistant material for the electrolytic solution, separator, gasket, or other constituent elements of the batter which does not impair the battery performance even when combined with the electrode. This makes it possible to provide a coin-type (button-shaped) non-aqueous rechargeable battery resistant to the reflow temperature.
In the case of a coin-type (button-shaped) non-aqueous rechargeable battery using a manganese oxide or lithium-containing manganese oxide or the positive electrode and a lithium-aluminum alloy or carbon doped with lithium either by contact doping or by electrochemical process for the negative electrode, in almost all of the combinations of the electrolytic solution and the heat-resistant battery members, the electrolytic solution and the lithium alloy undergo reaction during the reflow soldering is to cause an abrupt bulging or explosion, and an increase in internal resistance. In the case of using a lithium alloy negative electrode, there was a tendency of increasing the internal resistance because the negative electrode becomes unstable during the alloying or the doping of lithium.
Further, in case of sealing the positive electrode alone inside the coin battery together with the electrolytic solution, no explosion occurs even if it is heated at the reflow temperature; however, there was found a tendency to cause bulging of the coin-shaped casing. Presumably, the combination of a negative electrode containing active lithium and the manganese oxide or the lithium-containing manganese oxide accelerates the abrupt bulging or explosion, or the increase in internal resistance.
As a result of studying various types of oxides for the positive electrode, it has been found that, only in case of using a molybdenum oxide in combination with a negative electrode containing lithium in some form, particularly by using MoO3, a 3-V non-aqueous rechargeable battery can be stabilized specifically at the reflow temperature. The reason for this is not fully understood by principle, but the use of a molybdenum oxide for the positive electrode stabilizes the lithium-containing negative electrode at high temperatures as to considerably suppress the bulging of the battery and the increase in the internal resistance.
As the negative electrode, those which were extremely difficult for use in a battery produced by reflow process are now made usable. In case of using the negative electrode in a primary battery, metallic lithium can be used as it is. For secondary batteries, there can be used, for instance, a lithium alloy such as a lithium-aluminum, etc., or carbon doped with lithium, or a metallic oxide (e.g.e, SiO, WO2, WO3, doped with lithium.
Molybdenum oxide according to the present invention is stable at the reflow treatment temperature, and yet, it can reversibly occlude and discharge lithium.
Molybdenum oxides MoOx with x being in the range of from 2 to 3 (2 and 3 inclusive) can be used in the present invention. The potential becomes lower with x approaching 2, and is higher for x approaching 3. Although it is not fully understood by principle, the cycle degradation tends to become low with x approaching 2. Recently, there is a demand for a battery having wider range of charge voltage and of output power voltage due to the setting of the equipment supplied by the battery users. Since the battery voltage is determined by the potential difference between the positive electrode and the negative electrode, the charge voltage and the output power voltage can be controlled by changing the value of x of MoOx of the molybdenum oxide used for the negative electrode.
In case of carrying out the reflow soldering, it has been found that the use of a non-aqueous solvent having a boiling point of 200xc2x0 C. or higher at the ordinary pressure results in a stable process at the reflow temperature. Although there is a case that the reflow temperature increases to about 250xc2x0 C., no explosion of the battery occurred even when xcex3-butyrolactone (xcex3BL) having a boiling point of 204xc2x0 C. at the ordinary pressure was used for the electrolytic solution, probably because of the increase in pressure inside the battery. In the combination of the positive electrode and the negative electrode, it was found favorable to use propylene carbonate (PC), ethylene carbonate (EC), or xcex3-butyrolactone (xcex3BL), either alone or as a composite thereof.
Furthermore, a polymer may be used in addition to the organic solvent above. As the polymer, there can be used those generally used in the art; preferably used are, for instance, polyethylene oxide (PEO), polypropylene oxide, bridged polyethylene glycol diacrylate, poly(vinylidene fluoride), bridged polyphosphazene, bridged polypropylene diacrylate, bridged polyethylene glycol methyl ether acrylate, bridged polypropylene glycol methyl ether acrylate, etc.
As the impurities mainly incorporated in the electrolytic solution (non-aqueous solvent), there can be mentioned water and an organic peroxide (e.g., a glycol, an alcohol, a carboxylate, etc.). Each of the aforementioned impurities is believed to form an insulating film on the surface of the graphitized substance as to increase the surface resistance of the electrode. This signifies that there is a fear of negatively affecting the cycle life and the capacity. Furthermore, this may increase self discharge when stored at high temperatures (60xc2x0 C. or higher). Thus, it is preferred that the aforementioned impurities are reduced as much as possible in the electrolytic solution containing the non-aqueous solvent. More specifically, it is preferred that the water content is reduced to 50 ppm or lower, and the content of the organic peroxides is reduced to 1,000 ppm or lower.
As the supporting salts, there may be used one or more types of lithium salts (electrolytes) such as lithiumperchlorat (LiClO4), lithiumpresphohexafluoride (LiPF6), lithium borofluoride (LiBF4), lithium arsenic hexafluoride (LiAsF6), lithium trifluorometasulfonate (LiCF3SO3), bis(trifluoromethylsulfonyl)imidolithium [LiN(CF3SO2)2], or a lithium salt of, e.g., thiocyanate, aluminum fluoride, etc. In carrying out the reflow soldering, it was found that a supporting salt containing fluorine, for instance, lithiumpresphohexafluoride (LiPF6), lithium borofluoride (LiBF4) or lithium trifluorometasulfonate (LiCF3SO3), is more stable, both thermally and electrically, than a chloride-based salt, such as LiClO4. The content of the salt to be dissolved in the non-aqueous solvent is preferably in the range of from 0.5 to 3.0 mol/liter.
The solid electrolyte to be used in mixture with the polymer and the supporting salt is prepared by any method for solvent removal. More specifically, the method comprises, after dissolving the polymer and the supporting salt in acetonitrile or 1,2-dimethoxyethane, applying the resulting solution to the separator according to the present invention and drying. Otherwise, there may be employed a method comprising dispersing polypyrrole to a solution into which the supporting salt is dissolved, and removing the solvent thereafter. In case of a composite having a methacrylate ester as the framework (POE-PMMA), light may be used to irradiated the mixture of the monomer and the supporting salt to effect polymerization.
As the separator, there is used an insulating membrane having a large ion transmittance and a predetermined mechanical strength. As a material for use in the reflow soldering, glass fibers can be used most stable. However, there may be also used a resin having a thermal deformation temperature of 230xc2x0 C. or higher, such as polyphenylene sulfide, polyethylene terephthalate, polyamide, polyimide, etc. The pore size of the separator is set in a range generally used in the batteries. For instance, pores 0.01 to 10 xcexcm are used. The separator is used at a thickness generally used in the art for batteries, for instance, in the range of from 5 to 300 xcexcm.
For the gasket, generally used is polypropylene or the like. However, in case of carrying out reflow soldering, resins having a thermal deformation temperature of 230xc2x0 C. or higher, such as polphenylene sulfide, polyethylene terephthalate, polyamide, liquid crystal polymer (LCP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin (PFA), polyether ether ketone resin (PEEK), polyether nitrile resin (PEN), etc, have found to be free of explosions and the like at the reflow temperature, and yet, have found no problems such as a leakage of solution due to the deformation of the gasket even during the storage thereof after the reflow treatment.
In addition to the resins above, there can also be used polyether ketone resin (PEK), polyarylate resin, polybutylene terephthalate resin, polycyclohexanedimetylene terephthalate resin, polyether sulfone resin, polyaminobismaleimide resin, polyetherimideresin,orafluororesin. Amaterialcontaining 10% by weight or less of glass fibers, mica whiskers, fine ceramic powders, etc., is experimentally confirmed to realize a similar effect obtained in the present experiment.
As the method for producing the gasket, usable are an injection molding method, a thermal compression method, etc.
The injection molding method is the most generally known method for molding a gasket. However, in case of reducing the production cost at the sacrifice of molding precision, it is essential to assure air tightness by using a liquid sealing agent.
The thermal compression method comprises using, as a primary gasket molding, a sheet material thicker than the final product obtained as the product, and applying thermal compression shaping to obtain the final product.
In general, in case of elevating the temperature of a thermoplastic resin molding obtained by thermal compression method at a temperature not higher than the melting point of the material, the material tends to recover the shape of the primary molding. By taking advantage of the characteristics above, the interstice that is originally formed between the external and internal casings (which is made of metal) and the gasket (which is made of resin), or the loss of sufficiently high stress between the casings and the gasket, the non-aqueous rechargeable battery employing the gasket can be implemented with sufficiently high air tightness between the external and internal casings ((which is made of metal) and the gasket (which is made of resin) or with sufficiently high stress between the casings and gasket.
Furthermore, since the molding thus obtained tends to recover its shape with the passage of time, the same effect can be obtained in batteries obtained by a method not employing reflow soldering.
In particular, a gasket using tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin (PFA) prepared by the compression method, which comprises applying pressure to a sheet-like material while heating, was found to exhibit superior sealing properties as compared with such prepared by injection molding method. This is ascribed to the fact that PFA exhibits rubber elasticity, and that the product obtained by thermal compression tends to recover its original thickness at the reflow temperature as compared with the injection molded product which undergoes shrinking at the reflow temperature. Thus, the internal pressure at the sealed portion increases as to further achieve air tightness.
In case of a coin-type or button-shaped battery, an asphalt pitch, a butyl rubber, a fluorine-based oil, a chlorosulfonated polyethylene, an epoxy resin, etc., is used either alone or as a mixture thereof as a liquid sealing agent. If the liquid sealing agent is transparent, it is colored to clearly show the presence of coating. As a method for coating the sealing agent, there are used, for example, injecting the sealing agent to the gasket, coating the positive electrode and negative electrode casings, dipping the gasket into the solution of the sealing agent, etc.
In case the battery is shaped into a coin or a button, the electrode is shaped into a pellet by compressing the blend containing the positive electrode active material or the negative electrode active material. If the battery is shaped into a thin coin or button, the electrode may be punched out from a sheet-like shaped material. The thickness and the diameter of the pellet are determined depending on the size of the battery.
The pressing method of the pellet can be selected from the methods generally employed in the art, but particularly preferred is the pressing method using a metal mold. The pressure applied at the pressing is not particularly limited, but preferred is to apply a pressure in the range of from 0.2 to 5 t/cm2. The temperature during pressing is preferably set in the range from room temperature to 200xc2x0 C.
An electrically conductive agent, a binding agent, a filler, etc., may be added to the blended material for the electrode. The type of the electrically conductive agent is not particularly limited, and while allowing the used of a metallic powder, particularly limited are those based on carbon. A carbon material is the most common material, and used are, for instance, naturally occurring graphite (flaky graphite, scaly graphite, earthy graphite, etc.), artificial graphite, carbon black, channel black, thermal black, furnace black, acetylene black, carbon fibers, etc. As a usable metal, there can be used metallic powders and metallic fibers of, for instance, copper, nickel, silver, etc. Also usable are electrically conductive polymers.
The amount of carbon to be added is not particularly limited, and depends on the electric conductivity of the active material, the shape of the electrode, etc. However, in case of a negative electrode, carbon is preferably added at a concentration of 1 to 50% by weight, and particularly preferably, in an amount of 2 to 40% by weight.
The particle size of carbon is from 0.5 to 50 xcexcm in mean diameter, preferably from 0.5 to 15 xcexcm in mean diameter. By setting the particle size in a more preferred range of from 0.5 to 6 xcexcm in mean diameter, good contact between the active materials can be assured as to improve the network forming properties to assure electron conductivity, thus reducing the amount of active material not participating in the electrochemical reaction.
As the binding agent, although not particularly limited, preferred are those insoluble to the electrolytic solution. In general, used are those selected from polysaccharides, thermoplastic resins, thermosetting resins, polymers having the rubber elasticity, etc., either alone or as a mixture thereof; more specifically, such as polyacrylic acid or a neutralized polyacrylic acid, poly(vinyl alcohol), carboxymethyl cellulose, starch, hydroxypropyl cellulose, regenerated cellulose, diacetyl cellulose, poly(vinyl chloride), poly(vinyl pyrrolidone), tetrafluoroethylene, poly(vinylidene fluoride), polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene butadiene rubber, polybutadiene, fluorine rubber, poly(ethylene oxide), polyimide, epoxy resin, phenolic resin, etc. The amount of addition of the binding resin is not particularly limited, but preferred is to add in the range of from 1 to 50% by weight.
As the filler, there can be used any fibrous material so long as it does not undergo chemical change inside the constitution of the battery. In the present invention, used are fibers of carbon, glass, etc. The amount of addition of the filler is not particularly limited, but preferred is a range of from 0 to 30% by weight.
As the collector for the electrode active material, preferably used are metallic plates having low electric resistance. For the positive electrode, in addition to the materials such as stainless steel, nickel, aluminum, titanium, tungsten, gold, platinum, sintered carbon, etc., also used are aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, or silver. As the stainless steel, two-phase stainless steel is effective as an anti-corrosive material. In case of coin or button battery, nickel plating is applied to the side corresponding to the outer side. As a method of treatment, there can be used, for instance, wet plating, dry plating, CVD, PVD, cladding by pressure welding, coating, etc.
As the negative electrode, usable materials include, in addition to the materials such as stainless steel, nickel, copper, aluminum, titanium, tungsten, gold, platinum, sintered carbon, etc., aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, or silver, or an Alxe2x80x94Cd alloy. Usable as a method of treatment are, for instance, wet plating, dry plating, CVD, PVD, cladding by pressure welding, coating, etc.
The electrode active material may be fixed to the collector by using an electrically conductive adhesive. As an electrically conductive adhesive, there can be used a material obtained by adding, into a resin dissolved in a solvent, powders or fibers of carbon or a metal, or by dissolving an electrically conductive polymer into the solvent.
In case of an electrode shaped into a pellet, the adhesive is applied between the collector and the pellet electrode to fix the electrode. The electrically conductive adhesive mostly contain a thermosetting resin.
The usage of the non-aqueous rechargeable battery according to the present invention is not particularly limited; however, it can be used as a back-up power source of portable phones, pagers, etc., or as a power source of wrist watches equipped with generator functions, etc.
The battery according to the present invention is preferably assembled under a moisture-free atmosphere or under an inert gas atmosphere. Furthermore, the components to be assembled are preferably dried prior to the assembling. As a method for drying or dehydrating the pellets, sheets, and other components, there can be employed a method generally employed in the art. In particular, preferred is to employ methods using hot air, vacuum, infrared radiation, far infrared radiation, electron beam, or air reduced in moisture, either alone or as a combination thereof. The temperature is preferably set in the range of from 80 to 350xc2x0 C., and particularly preferred is the range of from 100 to 250xc2x0 C. The water content is preferably 2,000 ppm or lower with respect to the entire battery; and for the positive electrode blend, negative electrode blend, and the electrolyte, the water content of each of them is preferably set to 50 ppm or less from the viewpoint of improving the charge and discharge cycle properties.
Particularly effective is heating the pellet itself, and in this case, a temperature range of from 180 to 280xc2x0 C. is preferred. The proper duration of heating is 1 hour or longer, and the atmosphere can be selected from vacuum, air, or inert gas. The heating temperature must be set as such that it might be not lower than the reflow soldering temperature, and the heating condition must be determined by taking the strength of the organic binding agent into consideration. By thus heating each of the members to a temperature not lower than the reflow soldering temperature prior to their assembling, the occurrence of an abrupt reaction can be suppressed even if the battery is exposed to the reflow soldering temperature. Furthermore, by heating, the impregnability of the electrolytic solution into the pellets can be improved, resulting in an extremely advantageous effect in implementing a battery having improved battery characteristics as in the case of the present invention in which a highly viscous electrolytic solution is used at high temperatures.